The invention relates particularly to laminate precursors employed in the preparation of copper-clad assemblies having special utility in the production of high-resolution printed circuit boards and with a new method for making such laminate precursors.
This invention is related to the invention disclosed and claimed in copending U.S. patent application Ser. No. 227,290--Lifshin, et al., filed Jan. 22, 1981. The Lifshin, et al. application is directed to the method of depositing copper directly on a carrier surface by vapor deposition while controlling the adhesion between the copper and the carrier surface. This is accomplished by maintaining the carrier surface at a temperature in the range of from about 100.degree. C. to about 250.degree. C. with the result that the peel strength (i.e. the force required to separate these components) is to be between about 0.20 at 2.0 pounds/inch. Copper-clad assemblies utilizing the laminate precursors of this invention may advantageously prepared by the practice of the invention set forth in U.S. patent application Ser. No. 406,588--Green, et al., filed Aug. 9, 1982. According to the Green, et al. invention, a bonding system comprising a metal oxide layer and a coupling agent layer is used to interconnect the copper film of the laminate precursor to a resin-bonded, glass-reinforced substrate.
U.S. Pat. No. 3,969,199--Berdan, et al. is illustrative of the prior art patents utilizing an aluminum carrier sheet as a substrate for the deposition and handling of a copper layer. In Berdan, et al. the copper is deposited by electroplating and carrier sheet surface modification is relied upon to develop a desired peel strength.
It is a particularly important objective of this invention to be able to utilize commercially available aluminum foil as the carrier sheet in the preparation of the laminate precursor leading to the manufacture of copper-clad assemblies for ultimate usage in the preparation of printed circuit boards.
None of the commercially available foil strong enough to be used for this purpose is completely free of hydrocarbon contaminants. The contamination of aluminum foil begins during the rolling of aluminum billets to produce aluminum foil. The rolling is done in several passes each reducing the aluminum thickness by approximately 50 percent. Each such cold reduction pass is followed by a stress relief anneal to avoid tearing and pitting of the aluminum during the next reduction. Drawing lubricants are used during the cold reductions, the most popular being the combination of kerosene and mineral oil.
Fully soft aluminum foil is desired in many applications and in making foil for such applications the drawing lubricant can be burned off by heating to 300.degree. C. or even higher. However, in the case of aluminum foil for use as a carrier sheet, retention of the mechanical properties is important. Foil thicknesses employed as carrier sheets are of the order of 0.002 inch thick and must be strong enough to be used in roll form in an unrolling and rerolling operation carried on with a fixed tension. In order to avoid tearing, when such carrier sheet material is separated from the completed copper-clad assembly, it is preferable that the yield strength of the aluminum carrier sheet not be less than about 10,000 psi. This latter, therefore, limits the techniques employable for driving off the surface-contaminating hydrocarbons.
Commercially available chemically cleaned aluminum foil (defined hereinafter) is particularly attractive for this use, since this commercially available material retains the full hardness of unannealed severely cold worked aluminum foil. The problem remains, however, that aluminum foil so cleaned still retains on its surfaces a minimum of 0.3 .mu.g/cm.sup.2 of hydrocarbon contaminants, which is enough to rule out van der Waals adsorption as a reliable adhesional force between vapor deposited copper and the aluminum carrier sheet on which it is deposited. It is, therefore, necessary to develop other adhesional forces in order to provide sufficient peel strength in order to have the aluminum foil/copper layer laminate remain intact until it is desired to remove the aluminum by peeling it away. By the practice of this invention it becomes practical to develop such adhesional forces using this commercially available aluminum foil "as received".
Another method by which a suitable peel strength can be produced is that described in the Lifshin, et al. application referred to hereinabove providing that the hydrocarbon contamination on the chemically clean aluminum sheet (about 0.3 to about 1.0 .mu.g/cm.sup.2) is at the low end of the range. However, even in the practice of the Lifshin, et al. invention it is to be expected that, if the surface hydrocarbon contamination of the aluminum sheet is in the high end of the range for chemically clean aluminum foil, the laminate precursor produced may not have the desired peel strength. In such instances this invention offers an alternate to discarding such sub-standard laminate precursor, because it now becomes possible to render to this material a predetermined magnitude peel strength such that the force required to peel the aluminum foil from the ultra-thin copper will not be so great as to tear the copper film from its anchorage in the substrate or tear the aluminum foil or be a force so small that the aluminum foil falls off the ultra-thin copper during handling.